1. Field of Art
This invention relates to vertical axis wind turbines.
2. Brief Description of the Prior Art
The Darrieus-type vertical axis wind turbine (VAWT) having its rotating shaft transverse to the air flow was patented by G. J. M. Darrieus in the United States in 1931 (U.S. Pat. No. 1,835,018). The Darrieus-type vertical axis wind turbine is said to resemble an eggbeater with curved blades connected at both ends to the ends of the rotating shaft. Each blade of the turbine is a symmetric airfoil in cross section and is curved in the shape that a perfectly flexible cable of uniform density and cross section would assume if spun about a vertical axis. The shape of the blade is represented by the Greek word "troposkein" meaning turning rope. With blades having a troposkein configuration, major stresses are in tension when the rotor is spinning. Thus, rotation of the turbine will not cause the blades to bend nor produce bending stresses in the blades.
The operational principle of the vertical axis wind turbine (VAWT) is analogous to the aerodynamics of a wing (airfoil) as is described in a paper entitled The Vertical Axis Wind Turbine "How it Works", SLA-74-0160, December 1974 by B. F. Blackwell of Sandia Laboratories, Albuquerque, New Mexico. Fluid forces on the airfoil in a VAWT are divided into lift and drag forces with a component of the lift force causing rotation and a component of the drag force opposing rotation. The driving torque will be positive as long as the driving component of the lift force exceeds the opposing component of the drag force. With such a system, the aerodynamic performance is poor at low wind speeds and the blades may stall at low rotational speeds. Consequently, some type of auxiliary device such as a motor must usually be employed to start the system.
Although the Darrieus-type VAWT has been around for over fifty years, it has received little attention until the last two decades. In the 1960's, the National Research Council of Canada initiated a program to further develop and demonstrate Darrieus-type VAWTs, and their efforts have led to the installation of over two dozen turbines, including several in the United States. In 1973, Sandia Laboratories in Albuquerque, New Mexico began efforts to perfect VAWTs and two research machines are now in operation at Sandia's test site. Sandia's efforts are directed and funded by the United States Department of Energy through its Albuquerque Operations Office.
Interest in Darrieus-type vertical axis wind turbines has been stimulated in recent years by the energy crisis and by the recognition of the important advantages of such turbines which include: (1) the turbine accepts wind from all directions and therefore doesn't require costly direction orienting equipment, (2) the turbine doesn't require adjustment of pitch of the blades to limit maximum output at high wind speeds, (3) the generator and other working parts of the turbine are at ground level thereby obviating any need for a costly support tower, and (4) the blades of the turbine are stressed primarily in tension and therefore are lighter and less expensive.
Despite the advancements that have been made in the design of VAWTs in the last two decades and the inherent advantages of VAWTs, a VAWT is desired which can be easily erected at the site, which can be conveniently serviced and which will provide long dependable service at a reasonable cost.